1. Field of the Invention
The present invention relates to a technique of improving a proton conductive film of a fuel cell and, more precisely, to a proton conductive film comprising a porous substrate whose pores are filled with a polymer having a sulfonic acid group, a process for its production, a membrane electrode assembly and a fuel cell using it.
2. Background Art
In recent years, a fuel cell has been noted as a cell which imposes a less load on the environment and shows a high efficiency. In this fuel cell, a fuel such as hydrogen or methanol is electrochemically oxidized using oxygen to thereby provide energy as electric energy converted from chemical energy. As such fuel cell, various types have been developed, with a polymer electrolyte fuel cell using a proton exchange membrane as an electrolyte having been particularly noted.
FIG. 1 is a schematic diagram for illustrating the principle of the polymer electrolyte fuel cell.
In FIG. 1, numeral 1 designates a membrane electrode assembly which constitutes a main part of electricity-generating element. This membrane electrode assembly 1 comprises an electrolytic membrane 2 and a fuel electrode 3 (anode) and an oxidizer electrode 4 (cathode) joined on both surfaces of the membrane. A fuel path 5 is formed on the outer surface of the fuel electrode 3 so as to supply a fuel such as methanol. Also, an oxidizer path 6 is formed on the outer surface of the oxidizer electrode 4 so as to supply an oxidizer such as air.
The electrolytic membrane 2 constituting the membrane electrode assembly 1 is constituted by a proton conductive substance through which hydrogen ion can pass. In the fuel electrode 3, the fuel is oxidized by a catalyst contained in the fuel electrode 3 to generate electron, carbondioxide (CO2) and hydrogen ion. This hydrogen ion reaches the opposite electrode of the oxidizer electrode 4 and combines with oxygen of the oxidizer electrode 4 to generate water. On the other hand, electron generated in the fuel electrode 3 is taken out of this fuel electrode 3 and the oxidizer electrode 4 so as to provide electric power to an outer load circuit 7.
In the above-described fuel cell, a proton conductive membrane having a fundamental structure of a fluorine-containing resin such as perfluorocarbon sulfonic acid represented by Nafion (trade name) has conventionally been used as the electrolytic membrane 2. This perfluorocarbon sulfonic acid membrane shows an excellent proton conductivity. This excellent proton conductivity is exhibited through the cluster network of a water containing state. Therefore, in a fuel cell using methanol, a problem is generated that methanol permeated from the fuel electrode 3 (anode) diffuses through the cluster network of the electrolytic membrane to the oxidizer electrode 4 (cathode) to lower the output voltage. When this methanol cross-over phenomenon takes place, the supplied liquid fuel directly reacts with the oxidizer, thus energy not being taken out as an electric power. Therefore, there arises a fatal problem that a stable output can not be obtained.
In order to solve this problem, it has been known to depress swelling by introducing a cross-linked structure into the electrolytic membrane to thereby suppress methanol cross-over. However, this method involves a problem that cross-linking of the entire membrane leads to a serious decrease of proton conductivity.
On the other hand, Nafion (trade name) shows excellent properties, but is expensive. In order to widely spread the fuel cell, a more inexpensive proton conductive film showing excellent properties has been required to obtain. As a means for such purpose, it has been known to use a sulfonated polymer material in place of Nafion (trade name) (see JP-A-2005-113052 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). This technique is to sulfonate a polymer material film by using a sulfonating agent.
However, this technique of reacting a hydrophobic polymer with a highly polar solvent of sulfuric acid requires a long reaction time, and the reaction difficultly proceeds uniformly on the entire surface of the film. When the reaction time is prolonged or the reaction temperature is raised in order to remove the problem of this non-uniform reaction, influence of oxidization with hot concentrated sulfuric acid becomes so serious that there arises a problem of deterioration of a resulting porous membrane or a polymerization polymer. Further, there arises a problem that a slight change in permeability of sulfuric acid, treating temperature and treating period would cause a serious change in sulfonation degree, leading to serious inhomogeneity of characteristic properties as electrolytic membrane.
The present invention has been made for solving the above-mentioned problems with the proton conductive film, and provides an electrolytic membrane for fuel cell at a low production cost which causes less cross-over of methanol, and a membrane electrode assembly and a fuel cell using the membrane.